Catalyst

ABSTRACT

A catalyst comprising a plurality of catalyst gauzes that are arranged in series is provided. Each catalyst gauze is made of a first noble-metal-containing wire and a second noble-metal-containing wire which is embedded in the catalyst gauze and which gives the catalyst gauze a preferential direction. The catalyst gauzes according to the invention are arranged in series such that the angles between the preferential directions of neighboring catalyst gauzes are between 0° and 180°. The catalyst ensures a product yield that is reproducible over time and has a long service life.

BACKGROUND OF THE INVENTION

The present invention relates to a catalyst containing catalyst gauzesmade of noble metal. Such noble-metal catalysts are used, in particular,for gas reactions, such as in the manufacture of hydrocyanic acidaccording to the Andrussow process and in the manufacture of nitric acidaccording to the Ostwald process. In order to provide largecatalytically active surfaces for these reactions, such catalystsusually have spatial gas-permeable structures.

Often, these noble-metal catalysts consist of catalyst gauzes which aredesigned, for example, in the form of weft- or warp-knitted or wovenfabrics made of noble-metal wire. In addition to the bending strength,the ultimate tensile strength, and the ductility of the noble-metalwires, the wire diameter also plays a role in limiting the geometricshape of the catalyst gauze. Noble-metal wires having diameters rangingfrom 50 to 120 μm and ultimate tensile strengths ranging from 900 to1050 N/mm² are the only ones suitable, for example, for knitting wiresof specific platinum-rhodium, platinum-palladium-rhodium,palladium-nickel, palladium-copper and palladium-nickel-copper alloys.

As a result of these properties of noble-metal wires, other structuralcatalyst properties, such as the catalyst mass per surface unit and thenumber of meshes per surface unit, are also defined within certainlimits. For this reason, the reaction process to be catalyzed may onlybe optimized to a limited extent by means of these structural catalystproperties.

To provide for better adjustment of these structural catalyst propertiesin known catalyst gauzes, U.S. Pat. No. 5,669,680 proposes incorporatinga plurality of noble-metal wires having a helical, spiral-like structureinto the meshes during the manufacture of the catalyst gauzes. Such aprocess extends the planar two-dimensional gauze into the third spatialdimension. These gauzes are, therefore, also referred to as“three-dimensional gauzes.” Due to the helical structure, it is, forexample, possible to manipulate both the active catalyst surface and themass of the catalyst per surface unit via the thickness of theincorporated wire or via the number of spirals of the helical structure.The incorporated wires also facilitate the manufacture of heavythree-dimensional gauze systems. As a general rule, the incorporatednoble-metal wires extend linearly and in parallel to each other, thusalso giving the catalyst gauze a preferential direction that is definedby their extension.

Usually, a plurality of catalyst gauzes are installed in the reactionzone of a flow reactor, and are often arranged in series. A flow reactorfor catalytic oxidation of ammonia in which the catalyst is designed asa packing of a plurality of catalyst gauzes that extend in parallel toeach other is known from DE 602 01 502 T2. The packing is arranged inthe reaction zone in such a manner that the plane spread by the catalystgauzes extends vertically to the flow direction of a fluid containingthe reactants to be converted.

The arrangement of the catalyst packing extending transversely to theflow direction produces a flow resistance which, among other factors,depends on the porosity of the catalyst packing While the period ofapplication becomes longer, a decrease in porosity and an increase inflow resistance are often observed, whereby a uniform flow that isconstant over time is prevented and a reproducible average product yieldis impaired.

BRIEF SUMMARY OF THE INVENTION

The objective of the present invention is to provide a catalyst whichensures a high product yield that is stable over time and which,consequently, has a long service life. Specifically, the presentinvention relates to a catalyst comprising a plurality of catalystgauzes that are arranged in series. Each catalyst gauze is made of afirst noble-metal-containing wire and a second noble-metal-containingwire which is embedded in the catalyst gauze and which gives thecatalyst gauze a preferential direction. The catalyst gauzes accordingto the invention are arranged in series such that the angles between thepreferential directions of neighboring catalyst gauzes are between 0°and 180°.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the drawings:

FIG. 1 is a schematic of a catalyst according to a first embodiment ofthe invention, in which neighboring catalyst gauzes are arranged inseries with an orthogonally extending preferential direction;

FIG. 2 is a schematic of a catalyst according to a second embodiment ofthe invention, in which neighboring catalyst gauzes are arranged inseries with differing preferential directions;

FIG. 3 is a schematic of a catalyst according to a further embodiment ofthe invention, in which an intermediate gauze is provided betweenneighboring catalyst gauzes having different preferential directions;and

FIG. 4 is a schematic of a catalyst according to a further embodiment ofthe invention, in which an intermediate gauze is provided betweenneighboring catalyst gauzes having different preferential directions.

DETAILED DESCRIPTION OF THE INVENTION

Catalysts according to the invention contain a plurality of catalystgauzes that are arranged in series. Each of the catalyst gauzes is madeof a first noble-metal-containing wire and has a secondnoble-metal-containing wire embedded into the catalyst gauze. A“noble-metal-containing wire” or “noble-metal wire” may be understood todescribe a wire which consists of a noble metal or contains a noteworthycontent (>50 percent by weight) of a noble metal. The second noble-metalwire is embedded via a joint connection, preferably by sewing the secondnoble-metal wire to the catalyst gauze. As a result, the respectivecatalyst gauze acquires a three-dimensional structure. The embeddedsecond wire provides the catalyst gauze with a preferential directionwhich extends in both directions along the longitudinal axis of theembedded second wire. The arrangement of a plurality of catalyst gauzesin series is also referred to as “catalyst packing ”

The catalyst gauzes according to the invention are arranged in seriessuch that the angles between the preferential directions of neighboringcatalyst gauzes are between 0° and 180°. Thus, in contrast to knowncatalysts, the preferential directions in the catalysts according to theinvention include an angle unequal to zero, also referred to as“angular-offset arrangement.” Due to the offset arrangement of thecatalyst gauzes, high mechanical stability of the catalyst packing isachieved. This stability is of particular importance when the catalystgauzes are arranged transversely to the flow direction of a fluidflowing through the catalyst because the catalyst gauzes provide flowresistance. This results in the formation of a pressure upstream of thecatalyst—as seen in flow direction—which leads to a compression of thecatalyst gauzes or catalyst packing As a result, the gap volume insidethe catalyst packing is reduced. This gradually changes the flow of thefluid and affects the average product yield and the service life of thecatalyst.

The fact that the angle between the preferential directions ofneighboring catalyst gauzes may be unequal to zero counteractscompression of the catalyst gauzes. This is due to the fact that one ofthe causes for the increased compression of known catalyst gauzes withpreferential directions that is observed under pressure load is theirthree-dimensional structure. As seen transversely to the preferentialdirection, each of the catalyst gauzes alternately has recesses andelevations. If the preferential directions of neighboring catalystgauzes are arranged in parallel, these recesses and elevations extend inthe same direction, with the result that a complementary arrangement ofrecesses and elevations of neighboring catalyst gauzes is promoted if apressure is acting on the catalyst gauzes. Such a complementaryarrangement results in a comparatively high compression of the catalystgauzes, which is promoted by a continuous pressure load.

In contrast to a parallel alignment of the preferential directions, theformation of such a complementary structure of recesses and elevationsof neighboring catalyst gauzes is complicated in an angular-offsetarrangement. The increased mechanical stability of the catalyst packingthus obtained contributes to a prolonged service life of the catalystaccording to the invention.

In a preferred embodiment of the invention, the second wire has at leastone helically formed longitudinal section. The second wire may compriseone or a plurality of helically formed longitudinal sections or may beformed as a helically bent wire over its entire length. Due to the factthat the second wire contains at least one helically formed longitudinalsection, both the active catalyst surface and the mass of the catalystin relation to a surface unit of the catalyst gauze may be adjusted, forexample, via the wire thickness or via the number of spirals of thehelical longitudinal section. In a preferred embodiment, the second wirehas a helical structure over its entire length. Due to its uniformstructure, such a wire may be easily manufactured and processed. In afurther preferred embodiment, the helically formed and the linearlongitudinal sections of the second wire are alternating, with theresult that the catalyst properties may be adjusted by appropriatelyselecting the length of the particular longitudinal section.

Compared with the longitudinal linear section, the helical longitudinalsection has a more voluminous structure. When the preferentialdirections of neighboring catalyst gauzes are aligned in parallel, thehelical longitudinal sections of neighboring catalyst gauzes also extendin the same direction, with the result that the latter can slide intoeach other, also producing a compression of the catalyst gauzes. If,however, the neighboring catalyst gauzes have an angular offset, thiscompression effect is reduced.

In a further preferred embodiment of the invention, the second wire ismade of a platinum metal or of an alloy thereof. The term “platinummetal” includes platinum, iridium, osmium, ruthenium, rhodium, andpalladium. Platinum metals are used as catalysts, for example, in themanufacture of nitric acid according to the Ostwald process and in thesynthesis of hydrocyanic acid according to the Andrussow process. Theyhave adequate ductility as well as sufficient bending strength andultimate tensile strength to allow them to be processed to form catalystgauzes in the form of weft- or warp-knitted or woven fabrics.

Due to the fact that it is made of a platinum metal or of an alloythereof, the second wire has catalytic activity and is, over and abovethis, suitable for being embedded into the catalyst gauze because of itsmaterial properties.

Because the second wire is embedded into the catalyst gauze and shoulditself have catalytic activity if possible, a flexible noble-metal wirehaving a large surface is desirable. Since the flexibility and thesurface of the wire depend on the wire diameter, thin wires aredesirable as a general rule. It has therefore proven to be successful ifthe second wire has a wire diameter ranging from 40 μm to 100 μm. Asecond wire having a diameter of less than 40 μm can tear easily and iscomplex in its processing. On the other hand, wire diameters of morethan 100 μm have low flexibility.

In a further preferred embodiment of the invention, the angle betweenthe preferential directions of neighboring catalyst gauzes is between45° and 135°, which ensures that the mechanical stability of thecatalyst packing is particularly high.

In another particularly preferred embodiment of the invention, thepreferential directions of neighboring catalyst gauzes extendorthogonally to each other. This arrangement facilitates a particularlyeffective mechanical stabilization of the catalyst packing In addition,the catalyst may be manufactured easily and with a low loss in material.This benefit is particularly applicable if the catalyst gauzes have asquare shape. In this case, the orthogonality of the preferentialdirections of neighboring catalyst gauzes may be achieved by mutualrotation without any loss of material at the edges.

It has proven to be advantageous if the first noble-metal-containingwire is made of the same material as the second wire. Due to the factthat the first and second wires are preferably made of the samematerial, the composition of both wires may be optimally adjusted tomatch the process to be catalyzed. In this case, both wires have anoptimal catalytic activity.

With regard to maximizing the flexibility of the catalyst gauze made ofthe first noble-metal-containing wire, as well as to a surface of thenoble metal that is preferably large for catalyst applications, thefirst noble-metal-containing wire used for the manufacture of thecatalyst gauze is preferably as thin as possible. The first noble-metalcontaining wire preferably has a wire diameter ranging from 40 μm to 100μm, more preferably from 50 μm to 80 μm.

In another preferred embodiment of the invention, a two-dimensionalintermediate gauze that is made of a third noble-metal-containing wireis provided between neighboring catalyst gauzes. An intermediate gauzeincorporated between neighboring catalyst gauzes additionallycounteracts compression of neighboring catalyst gauzes under a pressureload. The intermediate gauze is a two-dimensional gauze which is, forexample, manufactured by textile processing methods, such as weft- orwarp-knitting or weaving of noble-metal-containing wires, and does nothave any preferential direction. Preferably, the intermediate gauze alsohas catalytic activity. The intermediate gauze preferably hasflexibility that is limited as compared with other gauzes. Preferably,the intermediate gauze has a mean mesh width ranging from 0.4 to 2.0 mm.An intermediate gauze having a mean mesh width of less than 0.4 mmgenerates a high gas resistance and therefore produces additionalcompression forces. Conversely, an intermediate gauze having a mean meshwidth of more than 2 mm has only a comparatively small catalyticallyeffective surface.

It has proven to be advantageous if the third noble-metal-containingwire is made of a platinum metal or of an alloy thereof and thus mayalso have catalytic activity. If the third noble-metal-containing wireis made of an alloy of a platinum metal, it preferably contains anoteworthy content (>50 percent by weight) of a platinum metal.

The flexibility of the intermediate gauze, for example, depends on thediameter of the intermediate gauze wire. In an advantageous embodimentof the catalyst according to the invention, the intermediate gauze wirehas a diameter ranging from 30 μm to 100 μm. If the wire diameter isless than 30 μm, the stabilization effect of the intermediate gauzewears off. Conversely, an intermediate gauze having a wire diameter ofmore than 100 μm has a small catalytically effective surface and hasonly a low catalytic activity per weight of the noble metal used.

FIG. 1 is a schematic representation of a first embodiment of a catalyst15 according to the invention, in which neighboring catalyst gauzes 1, 2are arranged in series. The catalyst is suitable for the manufacture ofhydrocyanic acid according to the Andrussow process. In the manufactureof hydrocyanic acid, a preheated gaseous ammonia-methane-air mixtureflows through the catalyst. The flow direction extends vertically to thegauze plane and is indicated by arrows 16, 17.

The catalyst gauze 1 has a basic structure 10 in the form of a textilefabric which was made by machine-knitting metal wires. The warp-knittedfabric has meshes with a mean mesh width of 0.8 mm. The metal wires aremade of a platinum-rhodium alloy (90/10) and have a diameter of 76 μmand a weight of 90.8 mg/m.

A plurality of flexible wires 3 are embedded into the two-dimensionalbasic structure 10 at regular spacings, with the result that athree-dimensional catalyst gauze 1 is obtained. The wires 3 are made ofa platinum-rhodium alloy (90/10) and have a wire diameter of 76 μm and aweight of 90.8 mg/m. In addition, the wires 3 are bent in the form of aspiral over their entire length, as shown in the enlarged detail 5. Theouter diameter of the spiral is, for example, 350 μm and the number ofspirals is 3 per mm. The available wire surface may, for example, beadjusted via the number of spirals per unit length. The catalyst gauze 1having wires 3 embedded into the basic structure 10 is particularlyheavy.

The basic structure 20 of the catalyst gauze 2 is manufactured in thesame way as the basic structure 10 of the catalyst gauze 1, with theresult that it has the same physical and chemical properties. Aplurality of helically bent wires 3 extending in parallel to each otherare embedded into the basic structure 20. The chemical composition ofthe wires 3 corresponds to that of the basic structure 10.

Due to the fact that the wires 3 that are embedded into the respectivebasic structures 10, 20 extend in parallel to each other, the catalystgauzes 1, 2 have a preferential direction which can extend in thedirection of the wires 3 and in the opposite direction. The preferentialdirections 6, 7 are indicated by arrows in FIG. 1. As shown in FIG. 1,the neighboring catalyst gauzes 1, 2 are arranged such that thepreferential directions 6, 7 extend orthogonally to each other (α=90°.

A second embodiment of the invention is catalyst 100 comprising aplurality of catalyst gauzes that are arranged in series, as shown inFIG. 2. Catalyst 100 also comprises a plurality of catalyst gauzes. Tosimplify matters, FIG. 2 shows only two neighboring catalyst gauzes 101,102. All neighboring catalyst gauzes of the catalyst are arranged inrelation to each other as shown in FIG. 2.

The catalyst gauzes 101, 102 are based on the same basic structures 110,120 which are manufactured by machine-knitting noble-metal-containingwires. Such noble-metal-containing wires are made of a PdNi5 alloy witha wire diameter of 80 μm. The weft-knitted fabric comprises meshes witha mean mesh width of 0.6 mm.

Noble-metal-containing wires 104, which are made of a palladium-nickelalloy (95/5), are embedded into the catalyst gauzes 101, 102. Thenoble-metal-containing wires 104 alternately have linear and helicallybent longitudinal sections. The length of the linear longitudinalsections is, for example, approximately 5 mm while the length of thehelically bent longitudinal sections is approximately 10 mm in theirbent condition.

The two catalyst gauzes 101, 102 differ only in that thenoble-metal-containing wires 104 are embedded into the basic structures110, 120 in different directions. The embedded wires 104 give apreferential direction to each of the respective basic structures 110,120. By way of example, the preferential directions 106, 107 are alsoindicated by arrows in FIG. 2. As shown in FIG. 2, neighboring catalystgauzes 101, 102 are arranged in series such that an angle (α) betweenthe preferential directions 106, 107 is 63°.

FIG. 3 shows catalyst 300 according to a further embodiment of theinvention, in which an intermediate gauze 303 is provided betweenneighboring catalyst gauzes 1, 2. These catalyst gauzes correspond tothe catalyst gauzes 1, 2 of FIG. 1 and are arranged such that theirpreferential directions 6, 7 extend vertically to each other. Anintermediate gauze 300 which is made by machine-knitting anoble-metal-containing wire and which has a mean mesh width that is lessthan that of the two catalyst gauzes 1, 2 is arranged between catalystgauzes 1, 2. Specifically, intermediate gauze 303 has a mean mesh widthof approximately 0.5 mm. The noble-metal-containing intermediate gauzewire is made of a PtRh5Pd5 alloy and has a wire diameter of 50 μm.

FIG. 4 shows a catalyst 400 according to a further embodiment of theinvention. In this catalyst, an intermediate gauze 403 is providedbetween neighboring catalyst gauzes 401, 402. The catalyst gauzes 401,402 are based on a two-dimensional basic structure 410, 420 in the formof a textile fabric which was made by machine-knitting noble-metalwires. The warp-knitted fabric has meshes with a mean mesh width of 1.0mm. The metal wires are made of a PtRh alloy 95/5 and have a diameter of61 μm and a weight of 60 mg/m. A plurality of flexible wires 430 areembedded into the two-dimensional basic structures 410, 420 at regularspacings, with the result that three-dimensional catalyst gauzes 401,402 are obtained. The wires 430 are made of a PtRh alloy (90/10) andhave a wire diameter of 70 μm and a weight of 77 mg/m.

The two neighboring catalyst gauzes 401, 402 are arranged such thattheir preferential directions 406, 407 extend at an angle (α) of 6° inrelation to each other. An intermediate gauze 403 made bymachine-knitting a noble-metal-containing wire and having a mean meshwidth of 0.8 mm is arranged between the two catalyst gauzes. Thenoble-metal-containing wire of the intermediate gauze 403 is made of aPtRh10 alloy and has a wire diameter of 76 μm.

Example

Compression tests were carried out in which a plurality of stacks eachcomprising 24 catalyst gauzes (L×W: 50 mm×50 mm) were subjected to aweight of 1 kg (weight force: 9.81 N) at an ambient temperature of1,100° C. over a period of 3 days.

The catalyst gauzes were made of a PtRh10 wire having a wire diameter of76 μm. A helically formed second wire made of PtRh10 was embedded intothe catalyst gauzes. The wire diameter of the second wire was 76 μm.Table 1 summarizes the specific arrangements of the gauzes and thecompression results that were measured. The values specified in the“Compression” column are relative to the initial stack height.

TABLE 1 Arrangement of neighboring catalyst gauzes CompressionPreferential directions: parallel (α = 0°) 40% Preferential directions:offset diagonally (α = 45°) 38% Preferential directions: offsetorthogonally (α = 90°) 36% Preferential directions: offset orthogonallywith 35% intermediate gauzes (α = 90°)

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim:
 1. A catalyst comprising a plurality of catalyst gauzesarranged in series, wherein each catalyst gauze comprises a firstnoble-metal-containing wire and a second noble-metal-containing wireembedded in the catalyst gauze, wherein the second wire defines apreferential direction of the catalyst gauze, and wherein the pluralityof catalyst gauzes are arranged in series such that an angle between thepreferential directions of neighboring catalyst gauzes is between 0° and180°.
 2. The catalyst according to claim 1, wherein the second wirecomprises at least one helically formed longitudinal section.
 3. Thecatalyst according to claim 1, wherein the second wire comprises aplatinum metal or an alloy thereof.
 4. The catalyst according to claim1, wherein the second wire has a wire diameter of 40 μm to 100 μm. 5.The catalyst according to claim 1, wherein the angle between thepreferential directions of neighboring catalyst gauzes is between 45°and 135°.
 6. The catalyst according to claim 1, wherein the preferentialdirections of neighboring catalyst gauzes extend orthogonally to oneother.
 7. The catalyst according to claim 1, wherein the first andsecond noble-metal-containing wires are made of the same material. 8.The catalyst according to claim 1, wherein the first wire has a wirediameter of 40 μm to 100 μm.
 9. The catalyst according to claim 1,further comprising at least one two-dimensional intermediate gauzecomprising a third noble-metal-containing wire, wherein the intermediategauze is provided between neighboring catalyst gauzes.
 10. The catalystaccording to claim 9, wherein the third noble-metal-containing wirecomprises a platinum metal or an alloy thereof.
 11. The catalystaccording to claim 1, wherein the third wire has a wire diameter of 30μm to 100 μm.